5.6 Further Acids & Bases Calculations (A Level only)

Explain, using equations, how chemicals in the blood resist changes in pH when hydroxide ions enter the bloodstream.

A buffer solution contains a mixture of ethanoic acid and its salt. A small amount of nitric acid is added to the buffer.

Write an equation, including state symbols, showing how this buffer can resist the change in pH.

Determine the pH of the buffer solution that is 0.050 mol dm^-3 of ethanoic acid and 0.050 dm^-3 of sodium ethanoate. You must show working and give your answer to 2 decimal places.

(The pK_a of ethanoic acid is 4.77)

Calculate the concentration of methanoic acid used in a buffer which has a pH of 3.45 and a methanoate salt concentration of 0.149 mol dm^-3. Give your answer to 3 significant figures.

(K_a of methanoic acid = 1.8 x 10^-4 mol dm^-3)

A 0.25 mol dm^-3 of sodium hydroxide solution, NaOH (aq), was added from a burette during a titration to 25.00 cm³ of a 0.15 mol dm^-3 solution of propanoic acid.

Calculate the pH of the solution after 10.00 cm³ of NaOH (aq) was added. Give your answer to 2 decimal places.

(K_a of propanoic acid = 1.34 x 10^-5 mol dm^-3)

Calculate the pH of the solution after 30.00 cm³ of NaOH (aq) was added.

Sorbic acid is used in food preservatives as a substitute for benzoic acid. It is highly effective in inhibiting the growth of mould which can spoil food and spread fatal diseases. 

The structure of sorbic acid is shown in Figure 1.

Figure 1

(K_a of sorbic acid = 1.7 x 10^-5 mol dm^-3)

25.00 cm³ of 0.020 mol dm^-3 of sorbic acid is titrated with 0.040 mol dm^-3 potassium hydroxide.

During the titration the potassium hydroxide solution is added to sorbic acid. 

What is the pH of a buffer solution made by dissolving 0.250 g of benzoic acid and 0.475 g of sodium benzoate in 250 cm³ of water? Give your answer to 2 decimal places.

(K_a of benzoic acid = 6.3 x 10^-5 mol dm^-3)

Calculate the pH of a buffer solution made by mixing together 120 cm³ of 0.15 mol dm^-3 ethanoic acid and 75 cm³ of 0.55 mol dm^-3 sodium ethanoate. Give your answer to 2 decimal places.

(K_a of ethanoic acid = 1.74 x 10^-5 mol dm^-3)

Calculate the new pH of the buffer solution given in part (b) if 1.00 cm³ of 1.00 mol dm^-3 sodium hydroxide is added. Comment on the pH value after the addition of sodium hydroxide.

Buffers are solutions that can resist changes in pH if a small volume of acid or base is added.

Figure 1 shows a sketch of a graph pH curve when ammonia, NH₃ (aq), is added to a hydrochloric acid, HCl (aq).

38.65 cm³ of NH₃ (aq) was required to neutralise 20.00 cm³ of 0.5 mol dm^-3 HCl (aq)

Figure 1

In order to find the K_a of a weak acid, a student carried out a titration. The method was carried out as follows:

Step 1 Transfer 15.00 cm³ of a weak acid to a conical flask.

Step 2 Fill the burette with 0.1 mol dm^-3 sodium hydroxide.

Step 3 Add 1 cm³ of sodium hydroxide from the burette into the beaker and take the pH reading using a pH probe

Step 4 Continue until the pH no longer changes

Step 4 Plot a graph of pH versus volume of base added

The student’s graph is shown below in Figure 2.

The student suggests that the pH value measured is the K_a of the acid. Is the student correct? Explain your answer.

Figure 2

Using Figure 2, calculate the value for K_a of the acid. Give your answer to 3 significant figures and include the units for K_a. 

Figure 3 shows the change in pH as hydrochloric acid is added to sodium carbonate.

Explain using equations the two reactions in this double end point titration.

Figure 3

This question is about buffers.

Give the meaning of the term buffer solution.

Buffers can be acidic but they can also be basic. This is determined by the reactants which are combined to create the buffer. 

Ethanoic acid is a weak acid. Hydrogen carbonate ions act as a weak acid if they are in an aqueous solution. 

Explain how the solution of ethanoic acid works as a buffer when small amounts of alkali are added.

A buffer solution contains 0.10 mol dm^-3 concentration of methanoic acid and a 0.20 mol dm^-3  concentration of sodium methanoate.

The values of K_a for methanoic acid is 1.80 × 10^–5 mol dm^–3.

Calculate the pH of this buffer solution at 298 K. Give your answer to 2 decimal places.

Propanoic acid has an acid dissociation constant value, K_a, of 1.34 x 10^-5 mol dm⁻³ at 298K.

A buffer solution containing propanoic acid and sodium propanoate was prepared.

The concentration of the propanoate ions in the buffer solution was 0.177 mol dm⁻³.  

The pH of the buffer solution was 4.01.

A vegetable extract can be used as an indicator in an acid-base titration. At pH 4 the extract is yellow and at pH 9 it is blue.

Suggest a type of an acid-base titration this could be used in. You do not need to name specific acids and bases.

Explain your choice.

A buffer solution was prepared with a pH of 8.0. A couple of drops of each indicator given in Table 1 were added to the buffer solution. The indicators do not react with each other.

State the colour that would be seen in the mixture of indicators and buffer solution.

Table 1

A student carried out a titration using hydrochloric acid and potassium hydroxide by adding small amounts of potassium hydroxide to 25.0 cm³ of 0.2 mol dm^-3 hydrochloric acid at 298K. The student plotted the results of the titration as a graph, see Figure 1.

Use Figure 1 to determine the volume of KOH added at the equivalence point of the reaction.

Figure 1

Suggest a suitable indicator for the reaction in Figure 1.

State the colour change which would occur during the reaction.

The student repeated the experiment, using 20 cm³ of 0.25 mol dm³ potassium hydroxide and 0.25 mol dm³ butanoic acid.

Butanoic acid is a weak acid. State the meaning of the term weak acid.

The student plotted a graph of the results of the reaction between butanoic acid and potassium hydroxide. The results are shown in Figure 2.

Use Figure 2 to determine the value of K_a for butanoic acid at 298K.

State the units for K_a.

Figure 2

This question is about buffers.

When we exercise, chemical changes occur which can cause our blood pH to drop. If the pH drops below 7.4 a condition called acidosis occurs. This can be very dangerous as many chemical reactions involve proteins which work at pH 7.4. The most important buffer for maintaining acid-base balance in the blood is the carbonic acid, H₂CO₃, hydrogencarbonate, HCO₃^- buffer.

Much of the atmospheric carbon dioxide we produce is absorbed by dissolving into the oceans, forming carbonic acid. An equilibrium is set up within the oceans using the same buffer solution as in the body, described in part (b).

In this buffer solution there is an almost unlimited amount of hydrogen carbonate ions and the carbonic acid available has a high concentration.

Using your answer to (b) part (i), suggest the changes which would lead to more CO₂ dissolving into the oceans. Explain your answer.

When we exercise, lactic acid CH₃CH(OH)COOH, may be produced by anaerobic exercise, if there is insufficient oxygen reaching the muscles.

The concentration of lactic acid is approximately 0.01 mol dm^-3 in the body directly after exercise.

The dissociation constant, K_a for lactic acid is 1.288 x 10^-4 mol dm^-3.

Give the expression for K_a and calculate the pH of lactic acid.